Current SGS cannot see a particular polar orbiting LEO Earth Observation (EO) satellite at each of its orbits around the earth (typically 14 orbits per day). It can happen that a LEO satellite is not able to exchange TT&C (Telemetry, Telecommand & Control) or receive tasking commands, or not able to download imagery data or sensor data during several consecutive orbits.
The number of visibility periods for a given SGS depends on the latitude of the SGS, typically an average of 5 contacts of 10 minutes each per day can be achieved from European latitudes (for example in Toulouse), and up to 10 contacts of 10 minutes each per day for a polar SGS (located for example in Sweden or in Svalbard).
However even with a polar SGS, or a network of SGS the situation is not perfect and faces many problems. These problems comprise non-technical problems of sovereignty using terrestrial means located in foreign countries.
Further there are problems of congestion. More and more EO satellites with increasing data streams are using the existing polar SGSs. It is expected that the next generation of satellites will congest the existing bandwidths, especially at the North Pole.
A further technical problem is that not all orbits are seen by the SGS. Therefore, a fixed response time cannot be guaranteed. Furthermore, the main polar SGSs are in the northern hemisphere, it is difficult to establish polar SGS in the southern hemisphere. As a consequence the southern part of the orbits is not used in an optimal way.
The prior art tries to solve these problems mainly by the following two approaches. Both approaches use data relay satellites. The first approach uses data relay satellites with radio frequency (RF) inter-satellite links while the second approach uses data relay satellites with optical laser inter-satellite links.
For relay satellites using RF inter-satellite links an antenna for tracking the LEO satellite is needed. This includes the need for a tracking mechanism with potential vibrations, life duration and weight problems. Further, this technique needs to move the antenna on the data relay satellite to acquire the line of sight, followed by a tracking of the low orbit EO satellite during the communication. Further it requires a large movement of the antenna to acquire the next EO satellite. A further problem is that these tracking antennas are generally large and can only be accommodated on rather big and expensive satellites.
For relay satellites using optical laser inter-satellite links the communication is established through an optical link. The communications need tracking at both extremities of the link, and involve in general light weight tracking mechanisms.
However, the models of optical laser terminals that will actually fly in the next coming years are both heavy and expensive, thus limiting the adoption of this technology to low orbit EO satellites with one or more of the following characteristics: large EO satellites, well funded/rich EO satellites programs, EO satellites with needs of transmission of large data files.
Another problem of optical laser technology is the time needed to acquire the line of sight between the geostationary satellite (GEO) and the low orbit EO satellite. This period of time can be as high as many tens of seconds. During this period there is no data communication between the data relay satellite and the low-orbit satellite. Thus, the communication efficiency of the system is reduced.
Further information to the prior art can be found in (1) European Data Relay System: The Space Segment, R. Giubilei, M Lisi and A. Morando Alenia SpazioSpA—Via Saccomuro, 2400131 Roma—Italy—AIAA-94-0906-CP; (2) The European Data Relay System: Present Concept and Future Evolution, Glullano Berretta, Agostino de Acostini, and Antony Dickinson—Proceedings of the IEEE, Vol. 78, No. 7, July 1990; and (3) Future Perspectives for the New European Data Relay System, M. Lucente, E. Re, T. Rossi, M. De Sanctis, C. Stallo, E. Cianca, M. Ruggieri, R. Winkler, A. Pandolfi.